Establishing a VTE Risk Score for Astronauts Algorithm

In April 2026, NASA’s Office of the Chief Health and Medical Officer (OCHMO) initiated a working group to review updated VTE case information, additional data gathered revealing altered blood flow status within a cohort of astronauts, and discuss progress of research and clinical activities intended to mitigate the risk of VTE during spaceflight with new evidence-based clinical practice recommendations.

The following is a summary of the working group’s recommendations:

• The working group’s conclusions emphasized that stasis in the left internal jugular vein (IJV) is consistently viewed as a dominant risk factor for VTE in microgravity, though there is active debate regarding the relative contributions of slow qualitative flow, endothelial factors and/or retrograde flow.
• Limitations of current in-flight ultrasound capabilities to accurately measure stasis/slow flow was cited as a concern, especially if using only stasis as a factor for indicating the need for prophylaxis.
• Additional review of assessing stasis with ultrasound in-flight was recommended.
• After reviewing the risk factors, additional discussions following the working group led to the majority of the panel agreeing that stasis and retrograde flow warranted use of prophylaxis.
• The working group also recommended assessing all other risk factors other than stasis to also determine when prophylaxis is warranted.
• Based on a literature review and summary of the panel contributions, a VTE Risk Score for Astronauts Algorithm was developed, which includes providing anticoagulation prophylaxis for stasis alone, or a combination of other thrombosis risk factors that are weighted by terrestrial literature.

NASA initially formed a working group in October 2024 after diagnosing venous thromboembolisms (VTEs) in astronauts during ISS missions. Experts reviewed case data, updated Clinical Practice Guidelines, and examined possible causes.